A new extended spectrum β-lactamase was detected in Serratia marcescens 42039 that was isolated from urine of patients with complicated urinary tract infection in Japan. This strain produced three different β-lactamase types (TEM-1, a cephalosporinase, and a new β-lactamase: CKH-1). The TEM-1 and CKH-1 encoding genes were conjugated from S. marcescens 42039 to Escherichia coli K-12 at frequencies of 10^-5 to 10^-6. The MICs of β-lactams against the transconjugant were: ampicillin >1600, piperacillin 800, cephalothin 1600, ceftazidime 6.25, cefotaxime 100, and ceftriaxone 200μg/ml. The CKH-1 enzyme was purified to more than 90% by ion-exchange chromatography. The molecular weight of purified CKH-1 was 30 K dalton and the isoelectric point was 8, 2, Relative Vmax/Km values (cephaloridme=100) of penicillin G, cephalothin, and oxyiminocephalosporins such as cefuroxime, ceftriaxone, and cefotaxime, were 256, 226, 116, 87, and 49, respectively. The I₅₀ values of tazobactam, BRL-42715, and clavulanic acid against CKH-1 enzyme were 0.0011, 0.0002, and 0.097μM, respectively. The enzymatic activity of CKH-1 was not inhibited by EDTA and anti-TEM-1 serum. These findings indicate that CKH-1 is a member of the groups of class A β-lactamases. This is the first report of a plasmid-mediated oxyiminocephalosporin hydrolyzing broad-spectrum β-lactamase from clinical isolates of S. marcescens.